The present invention relates to a circuit arrangement for evaluating the output signal of an active sensor which is configured as a binary current signal, i.e., a square-wave signal with two different current levels.
German patent application No. 39 36 831 discloses a circuit arrangement for conditioning the output signal of a rotational speed sensor. The circuit arrangement includes a trigger circuit or flipflop having change-over thresholds which are varied in response to a coupling factor that influences the output voltage of the sensor. A circuit arrangement of this type is appropriate for use in passive and active sensors. When evaluating the output signal of an active sensor which issues a binary current signal, however, a current/voltage conversion by way of a serially connected load impedance of a relatively low ohmic value would be necessary to provide a sufficient operating voltage for the active sensor. This impedance would require a great load capacity to withstand a short-circuit of the sensor line with respect to the supply voltage. In addition, a high degree of accuracy of the impedance would be necessary because the signal identification of the sensor depends on this accuracy. Impedances of this type are complicated, and the high energy consumption is a shortcoming. Identification of sensor errors would be possible only by additional circuit structure. Overlapping of the range of function and range of error identification is not possible with such a circuit arrangement when used in an active sensor.